Growth of Epitaxial Needlelike ZnO Nanowires on GaN Films Yung-Kuan Tseng, a Chih-Ta Chia, b Chien-Yih Tsay, c Li-Jiaun Lin, a Hsin-Min Cheng, a Chung-Yi Kwo, a and I-Cherng Chen a,z a Materials Research Laboratories and c Electronics Research and Service Organization, Industrial Technology Research Institute, Hsinchu, 310 Taiwan b Department of Physics, National Taiwan Normal University, Taipei, 100Taiwan Epitaxial needlelike ZnO nanowires were grown vertically over an entire epi-GaN/sapphire substrate at 550°C by low-pressure vapor phase deposition without employing any metal catalysts. A two-step oxygen injection process is the key of successful synthesis. The length of ZnO wires was up to 3.0 m. The diameters of the roots and tips of the ZnO nanowires were around 80-100 and 15-30 nm, respectively. X-ray diffraction showed the epitaxial orientation relationship between ZnO and GaN as  001 ZnO //  001 GaN along the normal to the plane, and  100 ZnO //  100 GaN along the in-plane direction, consistent with the selective area electron diffraction pattern taken at the ZnO/GaN heterointerface. High-resolution transmission electron microscopy confirmed that nanowire was a single crystal. A room-temperature photoluminescence spectrum of the wires revealed a low concentration of oxygen vacancy in the ZnO nanowires and showed high optical quality. © 2004 The Electrochemical Society. DOI: 10.1149/1.1825953 All rights reserved. Manuscript received June 14, 2004. Available electronically December 2, 2004. Semiconductor nanowires and nanorods have been attracting much attention in recent years, especially in mesoscopic research and the potential applications in manufacturing nanodevices. The main reasons include their interesting photonic and electronic prop- erties, and being able to be the important building block for inter- connects of transistors, junctions of metal-semiconductors, and the tips of emitters. Many studies have been carried out for Si and III-V systems. 1-5 However, research on the oxide systems, including SnO 2 , 6 SiO 2 , 7 GeO 2 , 8 ZnO, 9 ITO, 10 and Al 2 O 3 , 11 have just been conducted recently. Among them, ZnO is an intrinsic n-type semi- conductor with a wide bandgap 3.30 eV and a large exciton energy 60 meV. It conducts transparently and presents some interesting optoelectronic features, such as emitting short-wavelength light and room-temperature lasing. ZnO nanowires also could be used as good field electron emitters and chemical sensors because they have the characteristics of high-aspect-ratio structure, negative electron affin- ity, and chemical stability. Recently, optical and electronic devices based on 1-D ZnO nanostructure were also reported. 12,13 To apply ZnO nanowires in the optoelectronic field, they should be grown directly on substrates at the temperature below 550°C for the com- patibility of microelectronic manufacturing. Furthermore, they should be grown well oriented and could be patterned. Some efforts consisted with these requirements have been reported. Tseng et al. 14 demonstrated the feasibility of selected-area growth of ZnO nanow- ires at low temperature 500°C. Satoh et al. 15 have grown highly oriented ZnO whiskers on (001)  -Al 2 O 3 substrates at 550°C by chemical vapor deposition CVD. Huang et al. 16 reported the gas- phase synthesis of well-aligned ZnO nanowires on patterned Au catalyst with 110 sapphire substrates by the vapor liquid solid VLS reaction at 900-925°C. Park et al. 17 used metallorganic vapor phase epitaxy to grow ZnO nanoneedles vertically on Si111 at 400-500°C without metal catalyst. Growth on fused silica has also been reported by Wu et al. 18 Despite the successful reports of direct growth of ZnO nanowires on these common substrates, using low lattice mismatch buffer layer, especially epitaxial GaN, is a good way to deposit the ZnO nanowires on various material substrates. ZnO and GaN not only have a Wu ¨rtzite hexagonal structure with a low lattice mismatch  ( a ZnO - a GaN )/ a GaN = 1.6% , 19 but also have a small thermal mismatch (6.51  10 -6 K -1 for ZnO and 5.59  10 -6 K -1 for GaN. Furthermore, the use of a GaN buffer also offers the advantage of having the controllability of electrical conductivity in a broad range, which allow for greater flexibility in device design. Therefore, this combination is very appropriate to grow well-oriented ZnO nanowires on the buffer layers of various optical and electrical properties for further constructive applications. In our study, epitaxial needlelike ZnO nanowires were grown vertically over the entire GaN/sapphire at 550°C by low-pressure vapor phase deposition LPVPD without employing any metal cata- lysts. A two-step oxygen injection strategy results in the successful synthesis. The X-ray diffraction XRD-rocking curve and -scan techniques showed that the ZnO nanowires were grown epitaxially along the c -axis direction on the GaN/sapphire substrates. High- resolution transmission electron microscopy HRTEM was used to confirm that the nanowires were single crystals. The photolumines- cence PL spectra measured at room temperature indicated that the nanoneedles were of high optical quality. Experimental Our synthesis was performed by LPVPD. Zinc vapor source is Zn metal powder with the purity of 99.9% from Strem Chemicals. High-quality epitaxial GaN001 buffer layers of up to 2 m thick were grown on sapphire001 substrates by metallorganic vapor phase epitaxy MOVPE. The GaN/sapphire substrates and zinc va- por source in an alumina boat were inserted into the quartz tube and placed in the middle of the furnace. The distance between the Zn source and the substrate was 20 mm. The zinc vapor source was placed upstream. Both were located at the same horizontal level to make sure that they were in the same temperature region. A constant stream of argon flowed through the reaction system. A mechanical pump was used to evacuate the system to maintain the pressure inside the quartz tube at about 10 Torr. A programmable temperature controller was used to keep the heating ramp and fur- nace temperature in the range of 1°C. The heating ramp was set to be 20°C/min. The reaction gases were directed into reaction system in two steps by mass flow controllers. The first step was to lead the argon flow of 54 sccm into the reaction system as the experiment began. As soon as the furnace temperature reaching 420°C, oxygen flow of 0-3 sccm was added into the argon flow as the second step until the end of the experiments. The crystalline structure of the samples was analyzed using a Panalytical X’Pert MRD high- resolution triple axis X-ray diffractometer and a transmission elec- tron microscopy TEM; JEOL, JEM-2000FX, operated at 200 KV. The morphology and size distribution were characterized using a field-emission scanning electron microscope FESEM; LEO 1530, operated at 5 keV. A Jobin Yvon-Spex fluorolog-3 spectrophotom- eter was used to conduct the photoluminescence studies. Results and Discussion Our two-step oxygen injection strategy was proposed for inhib- iting the formation of Ga 2 O 3 . 20 Because of the different crystal structure, poor crystalline and large lattice misfits from ZnO and z E-mail: d837508@alumni.nthu.edu.tw Journal of The Electrochemical Society, 152 1 G95-G98 2005 0013-4651/2004/1521/G95/4/$7.00 © The Electrochemical Society, Inc. G95